Depletion Region Width Research Articles

The effect of kink and vertical leakage mechanisms in GaN-on-Si epitaxial layers

The connection between the kink position and vertical leakage mechanisms in GaN-on-Si epitaxial layers has been investigated. Based on combined experimental data and TCAD simulation results, we demonstrate that band-to-band tunneling and Poole–Frenkel effect greatly affect the current-voltage behaviors of GaN-on-Si epitaxial layers. Band-to-band tunneling occurring in both GaN and AlGaN layers could change a kink position, and Poole–Frenkel effect happening at high biases increases the slope of J-V at the voltage region after the kink. On the other hand, we found that the kink position is related to the depletion region width at the AlN/Si interface by numerical simulation. When the connection between the kink and leakage mechanisms was considered, numerical simulation results are consistent well with the current-voltage-temperature experimental results in GaN-on-Si epitaxial layers.

Significantly enhanced photocurrent density in NiCo2O4/a-C/Si photoanode for water splitting

Achieving stable operation of photoanodes used as components of solar water splitting devices is critical to realizing the promise of this renewable energy technology. In this work, amorphous carbon films (a-C), NiCo2O4 (NCO) and NCO/a-C bilayer films as protective layers were prepared on the surface of silicon, respectively. By comparing these protective layers of Si photoanode, we find that the NCO film can effectively protect Si from corrosion. It can not only lower initial potential but also increase photocurrent density. The NCO films were found to have the best catalytic performance. The photocurrent density of the NCO (60 nm)/Si photoanode is 185.6 mA/cm2 and that of the NCO (60 nm)/a-C (14 nm)/Si photoanode reaches high to 586.4 mA/cm2 at a voltage of 6 V vs SCE. The carbon interlayer reduces the width of depletion region built with n-Si, which further improves the current density of the NCO/Si photoanode. The photocurrent density of the NCO/a-C/Si photoanode is more than three times higher than that of the NCO/Si photoanode. The NCO/a-C bilayer film is an ideal protective layer of Si photoanode for photochemical water decomposition.

Ag alloying for modifications of carrier density and defects in Zn-rich (Ag,Cu)2ZnSnSe4 thin film solar cells

Abstract The Ag alloying Cu2ZnSnSe4 absorber has profound influences on the material properties including the carrier density, defects, grain structure and morphology, etc. It is found that Ag alloying could effective decrease the carrier density of Ag alloyed (Ag,Cu)2ZnSnSe4 (ACZTSe) absorbers and thus increase the depletion region width (DRW) of corresponding kesterite solar cells. At the same time, the CuZn related defects are decreased and consequently the band tailing and possible Fermi level pinning could be supressed. In addition, further adjusting [Zn]/[Sn] ratios in Zn-rich ACZTSe absorbers could be helpful to the performance of solar cells due to a fine modification of the carrier density and defects. After optimizing of chemical compositions by combining both modifications, the Ag alloyed Zn-rich ACZTSe based kesterite thin film solar cells have achieved the highest efficiency of 8.68%, in which the Ag alloying content with respected to [Ag]/([Ag]+[Cu]) and the [Zn]/[Sn] ratio are 10% and 1.14, respectively.

Electrical Behavior of Effects LCE and PAMDLE of the Ellipsoidal MOSFETs in a Huge Range of High Temperatures

Introduction: There is a growing interest in new electronic systems that must operate at high temperature (T) for automotive and space electronics, etc. To reach this purpose new device architectures have been proposed, such as the Gate-All-Around, double-gate, Multigate SOI MOSFETs, etc. [1]. However, there is a new concept, still little explored by the integrated circuits industries, which is based on the simple change of the gate geometry of MOSFETs [2]. This pioneering technique is capable for boosting the electrical performance of MOSFETs, thanks to two new effects of these structures, called “Longitudinal Corner Effect” (LCE) and “PArallel connection of MOSFETs with Different channel Lengths Effect” (PAMDLE) [2]. The Ellipsoidal layout style for MOSFET (EM) [4] is an example this new concept. Thus, this manuscript performs a comparative study by three-dimensional (3D) numerical simulations of the effects caused by the high T (300K-573K) in main electrical parameters of the EM in relation to the Rectangular MOSFET (RM) counterpart, regarding the same gate area, channel width and bias conditions. Furthermore, for the first time, is analyzed the electrical behaviors of LCE and PAMDLE in these conditions. Based on these results, we conclude that the EM always presents a better electrical performance {two times higher for the drain current normalized [IDS/(W/L)] and 77% higher for the transconductance normalized [gm/(W/L)]} than that found in the RM counterpart at high T. Besides, the LCE and PAMDLE always keep actives in the EM as the T increases. Results: The 3D numerical simulations were performed by using of the Sentaurus TCAD/Synopsys. The doping concentrations of the drain/source and channel are 1019cm-3 and 1017cm-3, respectively, gate thin-oxide thickness is 4.2nm and work-function of polysilicon is 4.15eV. The threshold voltages for both devices are equal to 50mV in room T.Fig.1 shows the IDS/(W/L) as a function of gate voltage (VGS) for different temperatures of the EM (Fig.1a) and RM counterpart (Fig.1b). Based on results, we observe that the Zero Temperature Coefficient (ZTC) biases of the EM and RM (VZTC) are practically similar (difference of 6%), while the EM IDS/(W/L) in the ZTC point is 142% higher than the one measured of RM, thanks the LCE and PAMDLE present in the EM structure.Fig.2 illustrates IDS/(W/L) as a function of drain bias (VDS) considering different temperatures. The EM IDS/(W/L) in all temperatures studied, considering the triode and saturation regions, are always higher (2 times higher) than the ones observed in RM counterpart.Based in these previous results, Fig.3 presents the gmmax/(W/L) and IDS_SAT/(W/L) as a function of the T. We have observed that the gmmax/(W/L) (72% and 81% for 300K and 573K, respectively, Fig.3a) and EM IDS_SAT/(W/L) (2 times higher, Fig.3b) are always higher than those found in the RM counterpart thanks to LCE and PAMDLE and they reduce with the T increases. Therefore, we have concluded that the LCE and PAMDLE are always active in this huge range of high temperatures, according Fig.4, which the resultant longitudinal electric field (RLEF) in the center of the channel length (L) of the EM (Fig.4a) reduces 6% as the T varies (300K-573K). The maps of the EM RLEF lines are illustrated in Fig.4b for different 2 Ts, wherein the width of the depletion region (d) composed by the source/channel regions reduces as the T increases. Furthermore, Fig.4c illustrates de map of the EM IDS lines which are curves near to the edges and straight in the center of the L, due to the interactions of the longitudinal electric field components (3 in the straight defined by the foci of the ellipse and 2 out of it). Conclusion: We have concluded that the LCE and PAMDLE are always actives as the T increases and consequently are capable of boosting the EM electrical performance in comparison to the one of the RM counterpart, regarding all T studied. Furthermore, as the T increases, the magnitude of the RLEF reduces 6%, because of the reduction of the Fermi Potential that defines the surface potential. Besides, we observe that the width of the region where the RLEF acts along the channel region of the EM increases due to reduction of depletion region width between source/channel regions (from d1 to d2), as the T increases. Acknowledgment: We thank to the São Paulo Research Foundation (FAPESP), grant #2017/10718-7 and CNPq, grant #308702/2016-6 for financial support.

Simulation and Optimization Design of SiC-Based PN Betavoltaic Microbattery Using Tritium Source

In this paper, the Monte Carlo method and numerical model are used to build the electrical model of a SiC-based betavoltaic microbattery using a 3H source, and the influences of structural parameters and the surface recombination effect on the output characteristics of the SiC PN battery are simulated. According to Monte Carlo calculations based on the energy spectrum of the 3H source, the ionization energy deposition approaches the exponential distribution along the depth direction, and most of the 22rs are concentrated near the material surface. The ionization energy deposition data is converted into non-equilibrium carrier information for the numerical simulation of the battery’s output characteristics. The simulation results show that the conversion efficiency of the battery rises first, and then decreases with the increase of the doping concentration of the N region. This is because the N region-doping affects the depletion region width and the built-in electrical potential at the same time. After considering the surface recombination effect, the conversion efficiency decreased significantly. Thinning the thickness of or moderately reducing the doping concentration of the P region will weaken the surface recombination effect.

2D modeling of silicon optical PN phase shifter.

In this paper, a 2D model of a silicon lateral PN optical phase shifter is presented, which can be used for multiple-parameter study and optimization of device performance without the need for any commercial numerical tools. The model shows good agreement with technology computer-aided design (TCAD) simulation and can be used to calculate the phase shift, absorption loss, modulation efficiency, and insertion loss of the phase shifter. Multiple-parameter study includes the waveguide dimensions, operating wavelength, cladding material, doping concentrations, junction offset, and applied voltage. The model employs the effective index method to determine the mode properties and construct the 2D mode field. The PN diode is modeled by taking into account the fringing electric field at the core-cladding interface, which results in a wider depletion region near the interface. Multiple-loss components are discussed, and the scattering loss and free-carrier absorption are modeled using Payne-Lacey and Soref models, respectively. The model uses 2D modal overlap with 2D carrier distribution across the waveguide to calculate the phase shifter performance metrics. The algorithm used to model the 2D nature of the PN diode depletion region is presented in detail and uses mathematical and analytical formulas instead of numerical methods, making the model faster and easy to implement, with accuracy on par with commercial tools.

Reliable anatase-titania nanoclusters functionalized GaN sensor devices for UV assisted NO2 gas-sensing in ppb level

Internet of Things applications require ultra-low power, integrable into electronic circuits and mini-sized chemical sensors for automated remote air quality monitoring system. In this work, a highly sensitive and selective detection of nitrogen dioxide (NO2) has been demonstrated by functionalizing gallium nitride (GaN) submicron wire with titania (TiO2) nanoclusters. The two-terminal GaN/TiO2 sensor device was fabricated by top-down approach. The photo-enabled sensing makes it possible to operate this sensor at room-temperature, resulting in a significant reduction in operating power. The GaN/TiO2 sensor was able to detect NO2 concentrations as low as 10 ppb in air at room temperature (20 °C) with a quick response-recovery process. The sensor was found highly selective toward NO2 against other interfering gases, such as ethanol (C2H5OH), ammonia (NH3), sulfur dioxide (SO2), methane (CH4) and carbon dioxide (CO2). Furthermore, principal component analysis has been performed to address the cross-sensitive nature of TiO2. The sensor device exhibited excellent long-term stability at room temperature and humidity and was quite stable and reliable at various environmental conditions. Continuous exposure of the device to siloxane for a one-month period has shown a very small degradation in sensor response to NO2. Finally, interaction of NO2 gas molecules with the GaN/TiO2 sensor has been modeled and explained under the light of energy band diagram. The photoinduced oxygen desorption and subsequent charge transfer between TiO2 nanoclusters and NO2 molecules modulate the depletion region width within the GaN, thus contributing to a high performance NO2 gas sensing.

Simulation of GaN micro-structured neutron detectors for improving electrical properties**Project supported by the National Natural Science Foundation of China (Grant Nos. 11675198, 11875097, 11975257, 61774072, 61574026, and 61971090), the National Key Research and Development Program of China (Grant Nos. 2016YFB0400600 and2016YFB0400601), the Fundamental Research Funds for the Central Universities, China (Grant No. DUT19LK45), the China Postdoctoral Science Foundation (Grant No.

Nowadays, the superior detection performance of semiconductor neutron detectors is a challenging task. In this paper, we deal with a novel GaN micro-structured neutron detector (GaN-MSND) and compare three different methods such as the method of modulating the trench depth, the method of introducing dielectric layer and p-type inversion region to improve the width of depletion region (W). It is observed that the intensity of electric field can be modulated by scaling the trench depth. On the other hand, the electron blocking region is formed in the detector enveloped with a dielectric layer. Furthermore, the introducing of p-type inversion region produces new p/n junction, which not only promotes the further expansion of the depletion region but also reduces the intensity of electric field produced by main junction. It can be realized that all these methods can considerably enhance the working voltage as well as W. Of them, the improvement on W of GaN-MSND with the p-type inversion region is the most significant and the value of W could reach 12.8 μm when the carrier concentration of p-type inversion region is 1017 cm−3. Consequently, the value of W is observed to improve 200% for the designed GaN-MSND as compared with that without additional design. This work ensures to the researchers and scientific community the fabrication of GaN-MSND having superior detection limit in the field of intense radiation.

Electro-opto-mechano driven reversible multi-state memory devices based on photocurrent in Bi0.9Eu0.1FeO3/La0.67Sr0.33MnO3/PMN-PT heterostructures.

A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro–opto–mechano-driven reversible multi-state memory device based on photocurrent in Bi0.9Eu0.1FeO3 (BEFO)/La0.67Sr0.33MnO3 (LSMO)/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructures. It is found that the short-circuit current density (Jsc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric–optical–mechanism integrated devices based on the BiFeO3-based heterostructures.

Analyses of Pinned Photodiodes With High Resistivity Epitaxial Layer for Indirect Time-of-Flight Applications

This article analyzes the performance of pinned photodiode (PPD) for Time-of-Flight(ToF) applications on high resistivity epitaxial wafers. Due to its wide depletion region, high epitaxial resistivity PPDs feature some key advantages in collecting photogenerated charges. This article compares the demodulation contrast of the same pixel design on epitaxial wafers with different resistivity at near-infrared wavelength to analyze their performance. By comparing the simulated profile of pixels with different epitaxial resistivity, the characteristics of the PPD pixel on high resistivity epitaxial wafer when collecting photogenerated charges are concluded. This article also discusses the effects of using high resistivity epitaxial wafer with various design parameters. It is found that the use of high resistivity epitaxial wafer can greatly improve crosstalk performance, allow pixels to work at higher modulation frequencies, and enable large-sized pixels to have good demodulation capabilities.

Modeling dislocation-related leakage currents in GaN p-n diodes

Finite element analysis software was used to model and visualize two p-n junction models: one with a single threading dislocation (TD) and a control model without a dislocation. TDs are modeled as a Gaussian distribution of trap states with an FWHM of 5 nm localized around the r=0 line in a cylindrical coordination such that the linear trap state density was 1 trap/c-translation; this model allows the cylindrical symmetry of the c-plane GaN crystal orientation to be utilized to avoid more computationally intensive 3D models. It was discovered that the interaction of the charged dislocation region with the p-n junction had many notable effects. At zero bias, it was observed that the depletion region width (using the Depletion Approximation) and the maximum electric field were markedly reduced near the dislocation line. More significantly, an asymmetric reduction in the diffusion barrier for electrons (Vbieff=3.03eV) and holes (Vbieff=0.81eV) was observed due to the asymmetric nature of the dislocation band bending related to the doping. The asymmetric reductions in diffusion barriers persisted into VA=2.4V leakage case where asymmetric current profiles for electrons and holes were also observed. Lastly, the diffusion barrier reduction resulted in an additional Shockley-Read-Hall nonradiative recombination leakage caused by a high np-product and trap state density near the intersection of the dislocation with the junction.

Influences of Cu concentration on electrical properties of CZTSSe absorbers and their device performances

CZTS precursors with different Cu concentration deposited by magnetron sputtering were annealed under H2Se and Ar hybrid atmosphere to obtain CZTSSe absorbers. A series of characterization techniques were carried out to investigate the influences of Cu concentration on phase structures and electrical properties of CZTSSe absorbers. Results showed that device performance is very susceptible to the Cu/(Zn + Sn) ratio. The increase of Cu concentration in the absorbers will likely result in the formation of cooper-containing secondary phases, which can reduce short-circuit current so that deteriorating device performance. Moreover, the higher Cu concentration leads to the higher carrier density and the shorter depletion region width, resulting in the decline of devices performance. In the case of the sample with Cu/(Zn + Sn) ratio of 0.70 in the precursor, CZTSSe absorber can achieve an appropriate electrical property, a homogeneous morphology with micron-sized particles, and a pure phase structure without obvious detrimental secondary phases. The highest power conversion efficiency of 9.13% in CZTSSe solar cells was achieved in this work.

Analysis of an Approximated Model for the Depletion Region Width of Planar Junctionless Transistors

In this paper, we investigate the accuracy of the approximated analytical model currently utilized, by many researchers, to describe the depletion region width in planar junctionless transistors (PJLT). The proposed analysis was supported by numerical simulations performed in COMSOL Multiphysics software. By comparing the numerical results and the approximated analytical model of the depletion region width, we calculated that the model introduces a maximum RMS error equal to 90 % of the donor concentration in the substrate. The maximum error is achieved when the gate voltage approaches the threshold voltage ( V t h ) or when it approaches the flat band voltage ( V F B ) of the transistor. From these results, we concluded that this model cannot be used to determine accurately the flat-band and the threshold voltage of the transistor, although it represents a straightforward method to estimate the depletion region width in PJLT. By using the approximated analytical model, we extracted an analytical formula, which describes the electron concentration at the ideal boundary of the depletion region. This formula approximates the numerical data extracted from COMSOL with a relative error lower than 1 % . The proposed formula is in our opinion, as useful as the formula of the approximated analytical model because it allows for estimating the position of the depletion region also when the drain and source terminals are not grounded. We concluded that the analytical formula proposed at the end of this work could be useful to determine the position of the depletion region boundary in numerical simulations and in graphical representations provided by COMSOL Multiphysics software.

Effects of aluminum doping upon properties of cadmium sulfide thin films and its effect on ITO/CdS:Al/NiOx/Ni/Au diodes

We analyze the Al doping effect in CdS films by chemical bath deposition and how it impacts this doping effect in a heterojunction, for the case of ITO/CdS:Al/NiOx/Ni/Au. The results show that the doping effect of aluminum on the CdS films by chemical bath deposition does slightly affect the structural and morphological properties. The impact of the doping is related to the electrical properties; first, with a decrement in the resistivity from 108 to 107 Ω cm, the work function decreases from 4.7 to 4.3 eV, the bandgap and the valence band suffers a small increment of 0.1 eV. The change in the electrical properties impacts the diode behavior, and the increment in the work function leads to an increment of the Fermi energy level difference between NiOx and CdS:Al; this difference was reflected in a wider depletion region and a reduction of the leakage current.

An AlGaN/GaN HEMT by the periodic pits in the buffer layer

In this paper, an AlGaN/GaN high electron mobility transistor (HEMT) by the periodic pits in the end of the buffer layer (PPB-HEMT) is proposed. The main focus of this proposed structure is based on controlling the carrier concentration and uniformity of the electric field distribution in the two dimensional electron gas (2DEG) channel layer that improves the breakdown voltage. The carrier concentrations fall under the gate by making periodic pits in the end of the buffer layer in the PPB-HEMT structure. Then, the electric field value reduces across the 2DEG channel layer and the electric field obtains the critical value at a higher drain-source voltage. Accordingly, the breakdown voltage of the PPB-HEMT improves approximately 32% compared to a conventional HEMT (C-HEMT). Consequently, the maximum output power density of the PPB-HEMT increases by 21% compared to the C-HEMT. Also, the depletion region width across the channel under the gate increases in the PPB-HEMT, and the gate-drain capacitance of the proposed structure decreases by 77% compared to the conventional structure. Therefore, the cut-off frequency of the PPB-HEMT improves by 50% compared to the C-HEMT due to the reduction in gate-drain capacitance. In addition, the current gain, unilateral power gain, and the maximum available power gain of the proposed structure increase. Therefore, the use of PPB-HEMT is more appropriate than the C-HEMT in high power and high frequency applications.

Impact of proton irradiation with different fluences on the characteristics of InP/InGaAs heterostructure

ABSTRACTThe effect of traps to C–V and I–V plots of InP/InGaAs heterostructure with 3 MeV proton irradiation at different fluences has been discussed. After proton irradiation, the total reverse capacitance increases, which does not only include the variation of the depletion region width, but also the charging and discharging effect of traps. The total actual traps density NSS of InP/InGaAs heterostructure could reach 13 orders of trap density, which is from the peak under reverse bias. The forward current is dominated by recombination current at low voltage and by the tunneling current at high voltage. The tunneling current and trap-assisted tunneling current are dominant in the reverse current.

Ultrasensitive near-infrared photodetectors based on MoTe2 transistors with tunable photoresponse time

The efficient near-infrared light detection of MoTe2 transistors has been demonstrated. The fabricated MoTe2 phototransistors show good photoresponse performances under an illumination of a 915 nm laser. The photoresponsivity and detectivity can reach to 1479 A W−1, 8.2 × 1011 Jones, respectively. Moreover, the photoresponse time can be effectively tuned by modulating the gate–source voltage. This mechanism could be explained by the depletion region width in channel is related to the gate–source voltage, and the Schottky barrier height between the MoTe2 channel and Pt source/drain metal is extracted to further confirm it.

Comparative Study Between Partially and Fully Recessed‐Gate Enhancement‐Mode AlGaN/GaN MIS HEMT on the Breakdown Mechanism

In this paper, two enhancement‐mode (E‐mode) AlGaN/GaN recessed‐gate MIS devices with the in situ N2 plasma treatment, known as partially recessed‐gate metal insulator semiconductor (MIS) device and fully recessed‐gate MIS device, are designed. Among them, the partially recessed‐gate MIS device shows excellent characteristics that the threshold voltage of +3.5 V, saturation current density of 722 mA mm−1, peak transconductance of 212 mS mm−1, and the power figure of merit of 4.39 × 108 W cm−2 which is the highest value in E‐mode AlGaN/GaN MIS device with the threshold voltage greater than 3 V. Moreover, the current collapse and breakdown voltage of partially recessed‐gate MIS device and fully recessed‐gate MIS device are investigated, and Silvaco simulation is carried out to analyze the difference of breakdown mechanism between two structure devices. Through the analysis of five main breakdown leakage paths, we can get the conclusion that the factor causing the breakdown voltage difference between partially recessed‐gate MIS device and fully recessed‐gate MIS device is the difference in the width of depletion region of the two devices.

A comparative study on the electrical and dielectric properties of Al/Cd-doped ZnO/p-Si structures

We have reported on the electrical and dielectric properties of Al/CdxZn1-xO/p-Si structures. The cadmium-doped zinc oxide (CdxZn1−xO) thin films with various Cd dopants (x = 0.10, 0.20 and 0.30) were deposited on p-Si wafers via sol–gel spin coating method. The admittance (Y = Gm + iωCm) measurements were performed at 1 MHz. The C−2–V plots were used to extract the main electrical parameters such as the diffusion potential (VD), the concentration of acceptor atoms (NA), depletion region width (WD) and barrier height (ΦB). The experimental results reveal that the capacitance increases with higher Cd dopant concentration due to the presence of interfacial charges while an opposite behaviour is observed in conductance. The lower values of conductance in the sample with high Cd content can be attributed to increase in series resistance. The dielectric measurements also confirm the effect of Cd substitution in ZnO on the device performance.

Precise determination of polarization fields in c-plane GaN/AlxGa1-xN/GaN heterostructures with capacitance–voltage-measurements

Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance–voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 ± 0.7 MV cm−1), Al0.26Ga0.74N (2.3 ± 0.6 MV cm−1), Al0.34Ga0.66N (3.1 ± 0.6 MV cm−1), Al0.41Ga0.59N (4.0 ± 0.7 MV cm−1) and Al0.47Ga0.53N (5.0 ± 0.8 MV cm−1) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance–voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration.